The present invention relates to a method and apparatus for manipulating stacked sheets, especially X-ray films, and more particularly to improvements in a method and apparatus for converting a stack of superimposed sheets into a succession of discrete sheets. Still more particularly, the invention relates to improvements in a method and apparatus for transferring successive outermost sheets from a source of stacked sheets to a transporting unit which accepts discrete sheets one after the other and advances such sheets to a cassette or to another destination.
German Offenlegungsschrift No. 24 03 469 discloses an apparatus for conversion of a stack of sheets into a succession of discrete sheets. Successive outermost sheets of the stack are attracted by a row of suction cups which are mounted on a rod-shaped carrier. In order to remove the outermost (uppermost) sheet of a stack of superimposed sheets, the suction cups are lowered simultaneously at right angles to the plane of and against the leader of the outermost sheet. In the next step, the suction cups are tilted to thereby lift the leader of the outermost sheet and to thus permit air to penetrate between the underside of the outermost sheet and the upper side of the next-to-the-outermost sheet. The suction cups are thereupon lifted a short distance and tilted back so that their orientation matches that during initial attraction of the leader of the outermost sheet. In an additional step, the suction cups are caused to move the leader forwardly (in its plane) and into the nip of advancing rolls forming part of the sheet transporting unit. Suction in the cups is caused to collapse after the leader of the removed outermost sheet has entered the nip of the advancing rolls, i.e., the suction cups are then supposed to release the sheet.
A drawback of the just described conventional apparatus is that suction in the cups cannot be caused to collapse instantaneously, i.e., the suction cups continue to adhere to the leader of the freshly transferred or singularized outermost sheet for a certain period of time following entry of the foremost part (leading edge) of the transferred sheet into the nip of the rotating (driven) advancing rolls. This invariably entails at least some slippage of advancing rolls with reference to the leader and/or some slippage of the leader relative to the suction cups. Slippage of the leader relative to the suction cups and/or slippage of advancing rolls with reference to the leader entails damage to and/or defacing of the sheet, especially if the sheet constitutes a film, such as X-ray film. Furthermore, slippage of the advancing rolls relative to the leader can entail improper delivery of the sheet to the next station, i.e., the arrival of such sheet at the next station is delayed by an unpredictable interval of time whose duration depends on several variable parameters such as maximum suction in the cups, the surface finish of the sheets, the area of contact between each suction cup and the sheet, the finish of the peripheral surfaces of the advancing rolls, the force with which the rolls are urged toward each other, and/or others.